H O S T E D B Y

Progress in Natural Science

Materials International

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Progress in Natural Science: Materials International 25 (2015) 12–21

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Pulsed laser deposited Al-doped ZnO thin films for optical applications

Gurpreet Kaurn, Anirban Mitra, K.L. Yadav

High Power laser Lab, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India

Received 29 June 2014; accepted 1 December 2014Available online 21 February 2015

Abstract

Highly transparent and conducting Al-doped ZnO (Al:ZnO) thin films were grown on glass substrates using pulsed laser deposition technique.The profound effect of film thickness on the structural, optical and electrical properties of Al:ZnO thin films was observed. The X-ray diffractiondepicts c-axis, plane (002) oriented thin films with hexagonal wurtzite crystal structure. Al-doping in ZnO introduces a compressive stress in thefilms which increase with the film thickness. AFM images reveal the columnar grain formation with low surface roughness. The versatile opticalproperties of Al:ZnO thin films are important for applications such as transparent electromagnetic interference (EMI) shielding materials and solarcells. The obtained optical band gap (3.2–3.08 eV) was found to be less than pure ZnO (3.37 eV) films. The lowering in the band gap in Al:ZnOthin films could be attributed to band edge bending phenomena. The photoluminescence spectra gives sharp visible emission peaks, enables Al:ZnO thin films for light emitting devices (LEDs) applications. The current–voltage (I–V) measurements show the ohmic behavior of the filmswith resistivity (ρ)�10�3 Ω cm.& 2015 Chinese Materials Research Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Al:ZnO thin films; Transparent conducting oxide (TCO); Pulsed Laser Deposition (PLD); Optical properties; Photoluminescence (PL)

1. Introduction

Transparent conducting oxides are promising materials inthe class of optoelectronic devices due to their potentialapplications in gas sensors, piezoelectric transducers, lightemitting devices and in solar cells [1]. Transparent oxide ZnOis a n-type semiconductor, has a direct band gap of 3–3.37 eVand with a large exciton binding energy of 60 meV, more thanthe thermal energy at room temperature [2]. ZnO is widelyused because of its properties, nontoxic, less expensive andabundantly available on earth, chemically and thermally stable[1,3]. There are quite similarities between the optical propertiesand band structures of ZnO and GaN [4]. Both GaN and ZnOare direct wide band gap (Eg43 eV) semiconductors, trans-parent in the visible region, having wurtzite crystal structures,comparable lattice constants, with nearly same c/a latticeconstant ratios [5]. Both of the semiconductors owe defect

/10.1016/j.pnsc.2015.01.01215 Chinese Materials Research Society. Production and hosting bymmons.org/licenses/by-nc-nd/4.0/).

g author. Tel.: þ91 11332285652; fax: þ91 11332273560.ss: physgk@gmail.com (G. Kaur).nder responsibility of Chinese Materials Research Society.

related deep level visible photoluminescence (PL) emissions[5,6]. It is found that GaN is known to be a good material forthe fabrication of the optical devices such as light emittingdiodes (LEDs) and laser diodes (LDs) [7,8], due to direct bandgap in UV region, good optical and electronic transportproperties at room temperature [8]. The similarities in theproperties of ZnO and GaN indicate that ZnO is one of thepromising materials in optoelectronic applications [4].The electrical properties of ZnO are improved via doping

with trivalent metal cations (Group III elements: B, Al, andGa) [1]. Theoretical explanations show that group III elementsgenerally have higher valances and with smaller ionic sizesthan the host Zn2þ cation [9], thus extrinsic dopants substituteinto host Zn sites and provide an extra electron which results inelectrical conduction [10]. Among cation-doped ZnO films,Al-doped ZnO (Al:ZnO) has been intensively investigated inrecent years [1–4,9,10]. Earlier reports for Al:ZnO thin filmsdeposited by Pulsed laser deposition (PLD) demonstrated thelowest resistivity values of 8.5� 10�5 Ω cm with extremelyhigh carrier concentration values of 1.5� 1021 cm�3 [11].Due to the large value of optical transmittance and high carrier

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G. Kaur et al. / Progress in Natural Science: Materials International 25 (2015) 12–21 13

concentrations Al:ZnO film has recently attracted attention forapplications involving transparent electrodes in solar cells, flatpanel displays and transparent thin film transistors [12]. So farvarious physical [12] or chemical deposition techniques arebeing adopted to deposit Al:ZnO thin films [11,13]. In thecurrent study, we have successfully deposited Al:ZnO thinfilms of different thickness, via Pulsed laser depositiontechnique (PLD). PLD is a relatively simple and versatiletechnique to grow oxide thin films as compared to otherphysical vapor deposition techniques [14].

In the present study, we have chosen 2% Al-doping byweight in ZnO. Deposited thin films are found to be highlycrystalline, oriented along a particular plane without anyimpurity phase. The thin film formation either involves island(Volmer–Weber) growth or layer by layer (Frank–VanderMerwe) growth [15]. The properties of the thin film arestrongly influenced by its thickness. With the increase innumber of layers the particle size, their shapes and distributionare modified which effectively changes the morphologicalproperties and the stress in the thin films [16]. The graindistribution and surface roughness influence the optical andelectrical properties of Al:ZnO thin film [9,16]. The opticaltransparency (T) and the refractive index (n) are dependent onthe film thickness, which affects the optical band gap [16]. Wehave seen the effect of thickness of Al:ZnO thin films on thestructural, optical and electrical properties and focused on theiroptical applications. These thin films are conducting at roomtemperature, and having large transmittance in the visibleregion.

Table 1Optimized deposition parameters for Pulsed Laser Deposited Al:ZnO thin filmsare listed as.

Laser used Nd-YAG LaserLaser wavelength 355 nmLaser energy 150 mJ/pulsePulse repetition rate 10 HzNo. of laser shots 8000–20,000 in steps of 4000Target used Al:ZnO-targetGas used Oxygen (99.9% purity)Gas pressure 2.67 PaSubstrate GlassSubstrate temperature 350 1CTarget to substrate distance 35 mm

2. Experimental

2.1. Preparation of Al:ZnO thin films

In the present study, we have successfully deposited Al:ZnO, thin films on glass substrate in the presence of oxygengas (99.9% purity), using Pulsed laser deposition (PLD)technique Excel Instruments, India. The target of Al:ZnOwas prepared by the solid state reaction method, via mixingAl2O3 (99.998%, Sigma Aldrich) and ZnO (99.998%, SigmaAldrich) powders in stoichiometric ratios, which was 1:49 for2% Al-doping. The powders were thoroughly mixed andgrinded for 6 h. Polymer solution of polyvinyl alcohol(PVA) with concentration of 3% was used as the binder.These homogeneous powders were pressed into a disk, calledpallet of diameter 20 mm and thickness about 5 mm, byapplying 20 t force with a hydraulic press. This pallet wassintered at 1200 1C for 2 h in the air to form the PLD target ofAl:ZnO. During the sintering process of Al:ZnO target, tworeactions occurred. In the first reaction Al-dopant diffused intoZnO lattice to substitute Zn. In the second chemical reaction ofAl2O3 reacted with ZnO to form spinel phase of ZnAl2O4.These two chemical reactions are given as follows [17]:

ZnOþxAl3þ-Zn2þ1� xAl3þx OþxZn2þ ;

ZnOþAl2O3-ZnAl2O4:

The Al:ZnO target was mounted on a multi-target carrouselcontrolled by a stepper motor which allows different targets tobe sequentially exposed to the beam paths. The DC motorrotated the target continuously so that no groove formationoccurs on the target surface. The ablated plasma plume wassymmetric with respect to the target surface with cosn(θ)distribution where n can vary from �4 to 30. The depositionsystem was first evacuated up to the pressure of the order of10�4 Pa (10�6 Torr). An Nd:YAG (neodymium-dopedyttrium aluminum garnet; Nd:Y3Al5O12) laser operating atwavelength 355 nm and energy 150 mJ/pulse with fluence�2 J/cm2, was used to ablate the targets. The glass substrateswere initially cleaned in an ultrasonic bath and then washedwith acetone. Deposition parameters were optimized to obtainhighly oriented thin films. The films were grown at 350 1Cunder oxygen pressure of 2.67 Pa (20 mTorr). We havedeposited four thin film samples with identity A, B, C and Dkeeping number of laser shots 8000, 12,000, 16,000 and20,000 respectively, the deposition time was increased. Eachof the thin film sample (A, B, C and D) thickness increased.The optimized parameters, i.e. the temperature and pressure ofdeposition for these thin films are listed in Table 1. To improvethe crystallinity of thin films, we have done post depositionannealing for 20 min in a vacuum.

2.2. Characterizations of the thin films

Structural characterizations of these thin films were exam-ined by X-ray diffractometer model Bruker AXS D8 Advancedin θ–2θ mode, with CuKα radiation (λ¼0.154 nm). The sourceof X-ray operated at a power of 40 kV� 30 mA. The surfacetopography and roughness of the as grown films wereexamined by Atomic force microscopy (AFM) NT-MDT:Model NTEGRA, in semi-contact mode with the silicon nitridetip of radius 10 nm. The grain morphology and the chemicalcomposition from Energy dispersive analysis of X-rays(EDAX) of the thin film samples was examined by Fieldemission scanning electron microscopy, FESEM (Model:Quanta 200F FEG and FEI Netherlands) with a very highelectric potential of 20 kV. The transmission electron micro-scopy (TEM), was observed by TEM model FEI TECNAI G2S-Twin operated at voltage 200 kV. TEM observationswere performed in both the image mode and diffraction mode.

G. Kaur et al. / Progress in Natural Science: Materials International 25 (2015) 12–2114

The optical properties transmittance, absorbance and reflec-tance for these thin films were observed by JASCO Model V-650 spectrophotometer. The optical band gap was calculatedfrom the Tauc's plot (αhν)2 versus hν. The photoluminescence(PL) properties of the thin films were observed by SHI-MADZU-RF-5301PC, spectrofluorophotometer. The current–voltage (I–V) characteristics within voltage sweep of 0–1 V,using four point probe method were observed at roomtemperature using Keithley 2636A dual channel Source majorunit sealed in the Black Box and provided with a probe station.To study I–V characteristics, Ag top electrodes were depositedon the thin films using a shadow mask.

3. Results and discussions

3.1. Structural properties from X-ray diffraction (XRD)analysis

The θ–2θ XRD pattern of pulsed laser deposited Al:ZnOthin films is described by Fig. 1. The diffraction peaks areindexed by comparing the data with JCPDS card file no. 79-0208 and 01-1136. Diffraction pattern indicates that the thinfilms are highly oriented along (002) plane with preferred c-axis orientation and hexagonal wurtzite crystal structures. Lowintensity peaks correspond to planes (100) and (110) are alsorecorded. The lattice spacing d is calculated from Bragg's law

Fig. 1. XRD pattern describing the excellent crystallinity and c-axis orientedthin films of different thickness, from A¼487 nm to D¼964 nm deposited viaPLD. The intensity and width of the peak is changed significantly with the filmthickness.

equation [3]:

d¼ nλ

2 sin θð1Þ

Here n is the order of diffraction and taken as 1, λ is thewavelength of X-rays used¼0.154 nm, for Cu Kα target and θis the Bragg diffraction angle of the peak (002). For thehexagonal lattice, lattice spacing d is given by the followingrelation [18]:

1

d2¼ 4

3h2þhkþk2

a2

� �þ l2

c2ð2Þ

where h, k and l are the Miller indices of the plane, a and c arethe lattice constant for the hexagonal unit cell. Using Eq. (2)values for the lattice constants “a” and “c” are calculated. Aslight increase in the lattice constants for Al:ZnO thin films aredue to the incorporation of Al3þ ions in the interstitialpositions [1]. The change in lattice spacing, d with thethickness of films implies a strain occurs between atomicplanes. Strain, ε in the thin films along c-axis can be calculatedusing the equation [18]:

ε¼ cf ilm�cbulkcbulk

� �ð3Þ

Here cbulk is the unstrained lattice parameter measured forbulk ZnO and its value is 0.521939 nm. This strain in the thinfilms is caused by the combined effect of thickness and Al-doping in ZnO. Stress in thin films can be obtained using thebiaxial strain model [18]:

σf ilm ¼ �232:8� ε GPað Þ ð4ÞCalculated values of stress and strain in the thin films are

listed in Table 2. Small shifts in the position of the peak (002)also indicate stress in the films. The deficiency of crystallitesduring the growth can allocate the intrinsic stress. The intrinsicstress is induced in the thin films by the deposition parameterssuch as growth temperature, gas pressure, laser energy andpulse duration [19]. The negative sign with the value of stressdescribes the compressive intrinsic stress. There is an increasein stress with an increase in the film thickness. When the filmthickness is low the dislocation energy is high. The dislocationdensity and the stacking fault probabilities, all increase as thefilm thickness increases which increases the stress in the thinfilm. The crystallite size (l) is calculated from XRD data byusing the Scherrer formula equation [20]:

l¼ 0:9λβ cos θ

� �ð5Þ

Here β is the full width at half maxima of the peak (002).The crystallite size can be found to decrease with an increasein the thickness of thin films implies improvement in structuralproperties. Degree of texture of a particular plane (hkl) is givenby the ratio of intensity of that particular plane to the sum ofthe intensities of all the planes observed in the XRD pattern[21]. Calculated values of the crystallite size and the degree oftexture of the plane (002) for the thin films are listed inTable 2. The lattice parameters are modified with the change infilm thickness from sample A to D, which influences the stress

Fig. 2. AFM images describing the surface morphology for the Al:ZnO thin films. The grain distribution and the surface roughness are influenced by the samplethickness, from A¼487 nm to D¼964 nm.

Table 3Grain size of the thin films calculated from AFM and FESEM analysis,

Table 2Calculations from XRD analysis of thin films: Lattice spacing, lattice constants, strain, stress, crystallite size and degree of texture for plane (002). As the thicknessincreases from sample A to D, the lattice parameters are changed which influence the stress in the films.

Sample Lattice spacing, d (nm) Lattice constant, a (nm) Lattice constant, c (nm) Strain (ε) Stress (GPa) FWHM (β) Crystallite size (nm) Degree of texture

A 0.2663 0.3361 0.5326 0.0205 �4.7729 0.4794 17.12 0.8311B 0.2661 0.3359 0.5322 0.0197 �4.5946 0.5402 15.21 0.8121C 0.2651 0.3339 0.5302 0.0158 �3.6757 0.6201 13.24 0.9851D 0.2646 0.3326 0.5292 0.0139 �3.2475 0.6372 12.88 0.7882

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and strain in the films. The crystallite size and degree oftexture are also affected.

Average and Root mean square (rms) roughness calculated from AFManalysis.

Sample AFM FESEM

Grain size (nm) Roughness (nm) Grain size (nm)

Average rms

A 22.706 2.7827 2.9885 20B 20.834 2.1989 2.2422 16.67C 17.925 1.6752 1.7419 12.5D 15.005 1.1768 1.2587 10

3.2. Surface morphology and roughness analysis fromAFM images

AFM images for the surface morphologies of Al:ZnO thinfilms are described in Fig. 2. The surface of the films wasreasonably smooth with the roughness o5 nm. Grain growthwas dense with highly homogenous distribution as clearly seenin 3-D AFM images of the surfaces. In Al:ZnO thin filmscolumnar grain growth was faster than that in basal planes.Deposition pressure was low i.e. 2.67 Pa (20 mTorr), indicat-ing that high deposition kinetic energy which leads tocolumnar grain growth. Thus observed grains formed in acolumnar pattern, without the presence of any voids. The

observed grains had small diameter and the average grain sizevaried from 23 nm to 15 nm with the change in film thickness.The surface morphology of the thin films was highly

Fig. 3. Grain formation observed from FESEM analysis describes the changes evaluated, decrease in grain size with the increase in film thickness.

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dependent on conditions of ambient temperature and pressure.During the annealing process elevated temperature can stimu-late the migration of grain boundaries, thus result in thecoalescence of more grains. Grain growth also contributes toincreased surface roughness and larger microcracks [19]. Theobserved values of average grain size and roughness of the thinfilms are listed in Table 3.

3.3. Grain formation, EDAX and SAED from FESEM antTEM analysis

Grain formation in thin films analyzed by FESEM isdescribed by Fig. 3. It can be observed that the grains arefound to be smaller with size less than 20 nm, and morphol-ogies be dense and continuous. The grains are uniformlydistributed and get a good adhesion to the substrates. It isbelieved that annealing of the thin films provides a largedriving force for internal atomic diffusion, which leads to graingrowth. At high temperature atoms have more energy toacquire a correct site in the crystal lattice [19]. Thus, thegrains with low surface energy grow larger. The energydispersive analysis of X-rays (EDAX) describes the atomicand elemental composition of the thin films. EDAX detects theX-rays produced as the result of the electron beam interactionswith the sample. It has been found that the experimentallyobserved values of the atomic and the weight composition ofthe elements matched well with the calculated one. Fig. 4describes the EDAX spectrum, which displays a clear Al Kα

line at 1.5 keV indicates Al-doping. Doping of Al3þ ions inZn2þ increases the number of nucleation sites on the substrate.Smaller grains and dense film structure result from theincreased degree of preferential alignment [22]. The columnargrain growth enhances with the thickness and it reduces thegrain sizes. The grain sizes measured by AFM and FESEM arenot same, which is due to two main reasons. Firstly, theresolution of FESEM is better over AFM, the difference inresolution results in the variation in grain size. Secondly thedistinctive principle of measuring grain size by both thetechniques. The AFM develop the image by the electrostaticforces (attractive or repulsive) of interacting between samplesurface and the probe tip (Si3N4). A topographic image ofthe surface can be generated by rastering the probe over thespecimen surface and recording the displacement of thepiezoactuator as a function of position. The AFM records thesignal intensity and digitally process the film surface image.Thus, the estimation of grain size depends on the artifactsinvolved in image recording. The FESEM image involves theinteraction of specimen with the highly energetic electronbeam and the secondary electron emissions. FESEM showsthe proper grain boundary formation on the surface of thespecimen.Figs. 5 and 6 describe the TEM obtained nanostructured

grains and selected area electron diffraction (SAED) pattern ofthe deposited thin films respectively. TEM was utilized tocharacterize the microstructure of materials such as grain size,morphology, crystal structure and defects, crystal phases and

Fig. 4. Energy dispersive analysis of X-rays, EDAX analysis to describe the atomic and weight composition of thin films.

G. Kaur et al. / Progress in Natural Science: Materials International 25 (2015) 12–21 17

composition. It enables to calculate the lattice spacing d, fromthe rings of selected area electron diffraction (SAED) pattern.The observed diffraction pattern consists of concentric ringsthat imply the polycrystalline nature of the films. The latticespacing d can be calculated using the equation:

Lλ ¼ Rd ð6Þwhere L is the camera length 135 mm, λ is the electronwavelength and R is the radius of the ring measured fromthe central bright ring. For the accelerating voltage 200 kVcorresponding wavelength of the electron is 0.0027 nm.Calculated values of d from Eq. (6) match well with theXRD data analysis and thus SAED rings are indexed.

3.4. Optical measurements

Optical transmittance versus wavelength spectra of thesethin films is presented in Fig. 7, in the wavelength rangebetween 200 nm and 800 nm. These curves show a welldefined interference fringe pattern, indicating the smoothsurface of the thin films. Thickness of the thin films can becalculated from transmittance maxima and minima i.e. Tmaxand Tmin in the fringe pattern [23]. The transmittance minimumin the fringe pattern is given by this equation:

Tmin ¼6n2

ðn4þ3:25n2þ2:25Þ ð7Þ

Solving Eq. (7) we get the value of refractive index of thinfilms, ‘n’. Now substituting ‘n’ in the following equations:

2nt¼ hþ1=2� �

λmax; ð8Þ

2nt¼ hλmin ð9Þ

Here ‘h’ is the order of interference fringes. Value ofthickness of thin films, ‘t’, is calculated by solving the abovetwo equations.The values of thickness and refractive index of the thin films

are listed in Table 4. The refractive index, n, increases with thefilm thickness. The thin films are highly transparent in thiswavelength range with the value of transmittance �80–90%.Absorption edge shifts towards lower wavelength with theincrease of film thickness. All the thin films show a sharpabsorption edge below 400 nm, due to the fundamentalabsorption of ZnO. High optical transmission enables Al:ZnO thin film suitable for application as the window layer forsolar cell fabrication and transparent electromagnetic interfer-ence (EMI) shielding materials.The optical absorbance and reflectance were also studied in

the wavelength range between 200 nm and 800 nm, which isdescribed by Figs. 8 and 9 respectively. The reflectance valuesare found to vary from 10% to 20% in wavelengths rangingfrom 200 nm to 800 nm. The change in optical reflectancecould be due to the morphological change in films, as theaspect ratio of crystallites changes with the thickness. Theoptical absorbance varies between 0% and 10% in thewavelength range of 450–800 nm. In the UV region theabsorption is more than 90%. Below the energy band gapthe absorption is caused by the defects states. The absorptionedge shifts towards longer wavelengths with the increase infilm thickness indicating a decrease in the band gap. From thetransmittance data absorption coefficient α can be calculatedusing Lambert's formula [1,11]:

α ¼ 1tln

1T

� �ð10Þ

Fig. 5. The crystal formation in the thin films as viewed under transmission electron microscopy (TEM).

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Here t is the thickness of thin film, T are the opticaltransmittance values. Optical band gap for the thin films iscalculated applying the Tauc model, and the Davis and Mottmodel in the high absorbance region [3,24]:

αhν ¼ Dðhν�EgÞn ð11Þ

Here hν is the photon energy, Eg is the optical band gap, andD is the constant. For n=1/2 the transition data provide the bestlinear fit in the band-edge region, implying the transition isdirect in nature. The band gap is calculated using Tauc's plotby plotting (αhν)2 versus hν by extrapolating the linear regionin the graph to (αhν)2=0, as shown in Fig. 10. The lineardependence of (αhν)2 with hν indicates that Al:ZnO thin filmsare direct transition type semiconductors. The optical band gapof thin films lies in the range between 3.2 eV and 3.08 eV(Table 4). It was observed that with the increase in the filmthickness band gap decreases. Al-doping reduces the band gapof Al:ZnO thin films as compared to ZnO thin films(3–3.37 eV). It is observed that Al3þ ions create a donor levelbelow the conducting band resulting in band edge bendingwhich lowers the band gap [25,26]. The entire Al is notcompletely absorbed in the ZnO matrix, while some of the Alatoms may be rested on the surface of ZnO in the form ofAl2O3, which could give rise to the allowed states near theconduction band in the energy band gap. Similar results arereported in Sn-doped and In-doped ZnO where the oxides

appeared on the surfaces of doped ZnO as SnO2 and In2O3

respectively [26,27].

3.5. Photoluminescence (PL)

Photoluminescence spectra of thin films at an excitationwavelength of 325 nm are shown in Fig. 11. PL is a techniqueemployed to investigating the effects of impurity doping on theoptical properties. PL spectrum relies on the creation ofelectron–hole pairs by incident radiation and subsequent radia-tive recombination photon emission. Numerous material proper-ties such as chemical composition, structure, impurities, kineticprocesses and energy transfer contribute to the intensity andspectral content of PL peaks [28]. PL spectrum shows twoemission bands one UV emission band and the other visibleemission band corresponding to blue-green regions. UV emis-sion band at 381 nm has lower intensity as compared with broadvisible emission band with peaks at 452–504 nm. The UVemission peak is the characteristic emission peak for the bandgap of Al:ZnO thin films. These emission peaks in the visibleregion correspond to the transitions from various defect statessuch as O-interstitial (Oi), O-vacancy (VO), Zn-interstitial (Zni),Zn-vacancy (VZn), donor acceptor pair and surface states, calleddefect related deep-level emissions (Fig. 12). The green emis-sion peaks result from the radiative recombination of photo-generated holes with electrons at the singly ionized intrinsic

(110)

(002)

(100)

Fig. 6. Selected area electron diffraction (SAED) pattern showing diffractionrings obtained from polycrystalline thin films.

Fig. 7. The optical transmittance versus wavelength plots for the Al:ZnO thinfilms at room temperature. The fringe pattern is permuted when the thicknessof the film is changed from A¼487 nm to D¼964 nm.

Table 4Calculations for thin film thickness and optical refractive index, from the fringepattern observed in optical transmittance versus wavelength plot, band gapvalues calculated from (αhν)2 versus hν plot and resistivity values calculatedfrom room temperature I–V plots.

Sample RefractiveIndex, n

Thickness, t(nm)

Band gap(eV)

Resistivity(Ω cm)

A 1.7323 487.5 3.2 0.59� 10�3

B 1.8074 632.5 3.11 1.24� 10�3

C 1.7569 789.1 3.12 1.42� 10�3

D 1.7768 964.5 3.08 2.28� 10�3

Fig. 8. The optical absorbance versus wavelength plots for Al:ZnO thin filmsamples. As the film thickness is increased from A¼487 nm to D¼964 nm, ared shift in absorption spectra is observed.

Fig. 9. The optical reflectance versus wavelength plots for Al:ZnO thin filmsamples.

G. Kaur et al. / Progress in Natural Science: Materials International 25 (2015) 12–21 19

oxygen vacancies. The transitions from Zni and the extendedZni state to the valence band respectively, results in the emissionpeaks in the visible region [29]. The broad visible emissionmight be attributed to electronic transitions from the near-conduction band-edge to deep level acceptors and to transitions

from deep donor levels to the valence band [19]. For Al-dopedZnO, Al ions exist in the form of Al3þ and Zn ions in the formof Zn2þ , Al ions will consume the residual O ions and results indecreasing the concentration of interstitial oxygen in the Al:ZnOlayers. It has been observed that, Al doping leads to thereduction of concentration of oxygen vacancies according tothe reaction: Al2O3þVö-2AlZnþ3Oo

X. Al-doping facilitatesthe non radiative transition rate, in regard to the localization ofthe Al impurity states [30]. Visible photoluminescence peaksenable Al:ZnO thin films for optoelectronic device applicationssuch as LEDs.

3.6. Electrical properties

Room temperature current versus voltage (I–V) character-istics of thin films are shown in Fig. 13. The enhancedconduction in these films is due to the Al-doping, whichcreates donor impurities via the formation Al3þ ions in thecrystal and the resulting Al:ZnO n-type semiconductor. The

Fig. 10. Plots of (αhν)2 versus hν describing band gap for the semiconductingAl:ZnO thin films. The decrease in band gap is observed with the increases infilm thickness from A¼487 nm to D¼964 nm.

Fig. 11. The room temperature Photoluminescence (PL) versus wavelengthspectra of the thin films at excitation wavelength of 325 nm.

Fig. 12. Schematic layout of various energy states correspond to Photolumi-nescence emission peaks.

Fig. 13. Plot describing current–voltage (I–V) curves for the semiconductingAl:ZnO thin films at room temperature. The resistivity increases with theincrease in film thickness from A¼487 nm to D¼964 nm.

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conductivity is mainly controlled by the counterbalancebetween the presence of oxygen vacancies, which act asdoubly charged electron donors and structural (Zn) defects,which influence the carrier mobility within the thin film. Anincrease in the concentration of oxygen vacancies increases thenumber of free electrons, causing an abrupt increase in theconductivity [14]. The decrease in lattice strain improves theconductivity by increasing the mobility of charge carriers(σ¼neμ). The calculated value for resistivity of these thinfilms is given by this relation [28]:

ρ¼ πt

ln 2V

I

� �ð12Þ

Here t is the thickness of the thin film, the factor (π/ln 2)�Ris the sheet resistance (Rsh). The calculated values forresistivity (ρ) are listed in Table 4. It can be clearly seen fromthe I–V curves, as the thickness decreases current in the thinfilm increases, thus electrical resistivity (ρ) reduces. The

surface of the thin films is exempt from cracks and dislocationboundaries, which can disrupt the flow of electrons. Theenhanced conductivity also results from the better crystallinityof thin films because randomly oriented crystallites disrupt theflow of current. High degree of crystal orientation improves themobility by reducing the probability of scattering of carriers atgrain boundaries. Electron–electron and electron–impurityscatterings result in the improvement of conduction. Thusincreased conductivity is due to doping of Al, which increasesthe carrier concentration as well as the mobility of ions.Conductivity also affected by various deposition parameters,preparation technique and in situ annealing of films.

4. Conclusions

Al:ZnO thin films of varying thickness were successfullydeposited on glass substrates. The deposited thin films aresingle phased with preferential growth in (002) plane, along c-

G. Kaur et al. / Progress in Natural Science: Materials International 25 (2015) 12–21 21

axis. A compressive stress is produced in the thin films due tothe combined effects of thickness and Al-doping. Surfacetopography studies from AFM and FESEM images describethat the grains are uniformly grown on the film surface. Thesurface of thin films is reasonably smooth with small values ofaverage and rms roughness. The deposited thin films showgood optical characteristics, very high optical transmittancein the visible region, low optical reflection and exhibit a sharpabsorption edge at a wavelength below 400 nm. Al-dopingin ZnO lowers the band gap. The large values of opticaltransmittance enable application of Al:ZnO thin films fortransparent window layer of solar cells. Photoluminescence(PL) spectra show sharp peaks in the UV and visible regions.Due to visible emission peaks Al:ZnO thin films find opticalapplications such as LEDs and laser diodes. The thin filmsshow high electrical conductivities. The electrical conductionis mainly controlled by the Al-doping and the oxygenvacancies which increase the carrier concentration and theirmobility. The Al:ZnO thin films exhibit excellent optical andelectrical properties, and are applicable for optoelectronicdevices.

Acknowledgments

The authors wish to acknowledge Department of Scienceand Technology, India for their financial assistance ofINSPIRE fellowship Grant no. 8782-12-44 for this researchwork.

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